Nucleonics and Analytical Chemistry Symposium
Radiochemical Activity Analysis DAVID N. AUME, Massachusetts Institute of Technology, Cambridge, Mass. Radiochemical activity analysis is defined as the identification and determination of the radioactivity due to individual constituents in mixtures of radioactive materials. Physical methods of identification of individual radioactivities and chemical methods for the resolution of mixtures are described. Addition of a known amount of inactive carrier and determination of the efficiency of recovery of the isolation process permit short-cut methods to be used in quantitative determinations and greatly cut the time necessary for an analysis. IIethods for the rapid determination of radioactivity due to each of the fission elements have been developed and the procedure for zirconium activity is discussed in detail. Applications in radioactive tracer production and utilization are suggested.
R
Another useful characteristic of a radioisotope which may be applied to identification is the characteristic rate of decay usually expressed as the half-life. If the logarithm of the counting rate of a sample of a pure radioisotope is plotted against time, a straight line is normally obtained and the time necessary for the samplr t o decay to one half its original value may be estimated. A useful table of radioisotopes arranged according to half-lives has bwn compiled by Clark and Jones ( 2 ) . The component of a mixture of two or even three radioisotopes can be identified by purely physical means only if the radiations or half-lives differ sufficiently. The problem is analogous to the determination of the composition of binary and ternary mixtures by measurement of physical properties such as light absorption and index of refraction. If the mixture contains two radioisotopes with distinctly different maximum 0-energies, the aluminum absorption curve will reveal itself as a composite of the two. If
ADIOCHERIICAL activity analysis involves the identification or determination of the elements or compounds responsible for the radioactivity of mixtures. As with other types of cheniical analysis, activity analysis may be qualitative if it is desired merely to identify the elements or compounds responsible for the radioactivity, or it may be quantitative if the fraction of the total radioactivity due to a given constituent is to be determined. Activity analysis should be carefully distinguished from ordinary chemical analysis, for frequently the results of the two bear no relationship to each other. h typical rare earth tracer preparation if submitted for chemical analysis might be found to contain nothing but dilute hydrochloric acid, a trace of silica, and miscellaneous microimpurities. Subjected to radiochemical @-activityanalysis, the same sample might be reported as 58% yttrium, 40% cerium, and 2% lanthanum. Activity analysis is important for the identification of artificially produced radioactivities and the determination of the composition of mixtures of artificial and natural radioactivities. Analysis may be made for a-, @-, or 7-activities; the general principles involved are the same in each case except for the actual physical measurement of the radioactivity. QUALITATIVE ACTIVITY ANALYSlS
If a single radioisotope is believed t o be present in essentially pure form, it is often possible to identify it by purely physical methods. Each radioisotope has its own individual radiation and decay characteristics, and these may be measured with the relatively simple aDparatus used in ordinary radiochemical tracer work. With a-emitters, a measure of the a-ray energy may be obtained either by determination of the range of the a-particles in air or by electronic measurement of the pulse height in the conventional a-chamber (1). The energy characteristics of 8-emitting isotopes, bvhich are the ones most commonly used, may easily be determined by measuring the aluminum absorption curve of the radiation. By plotting the logarithm of the counting rate of a 8-emitting sample against the thickness of aluminum (expressed in milligrams per square centimeter), introduced between the sample and the Geiger counter, characteristic curves are obtained for the various radioisotopes (Figure 1). If external conditions have limited the nuniber of possibilities, it is often as easy to identify the isotope from the appearance of the absorption curves as to identify a compound as one of several by comparison of their ultraviolet absorption spectra. Numerical values for the maximum @-energy may be derived from these absorption curves by the simple and ingrnious method introduced by Feather (4, 5 ) . Gamma-emitters do not give absorption curves as characteristic as those obtained with @-emitters,but measurement of the absorption half-thickness in lead provides a means of approximately estimating y-energies. 322
ABSORBER
Figure 1.
M C I SQ CM
460 960 I4 40 Aluminum Absorption Curves for Typical Fission Product Radioactivities
323
V O L U M E 21, NO. 3, M A R C H 1 9 4 9
of inactive carrier for the element or compound in question, (3) chemical separation and purification, (4)isolation of the desired constituent in a form suitable for counting and for the determination of the percentage recovery (chemical yield), (5) counting, and (6) calculation of the results. It is north while to consider the significant features of each step in some detail. Sampling. A s in other forms of chemical analysis, the method of taking the sample is of great importance and the obtaining of a representative sample is often the most difficult step in the entire analysis, particularly if the radioelement sought is given to forming colloidal suspensions. The so-called “carrier-free tracers” may contain the radiotlement in concentrations of the order of 10-12 molar, and such small amounts of material may easily be lost by adsorption on the walls of the container, traces of silica, and dust. Barium tracer in dilute hydrochloric acid, for example, is stable, but the same tracer in dilute sulfuric acideven though the solubility product of barium sulfate is not exceeded-niay yield very uncertain r f d t s due to adsorption of the barium on the walls of the sample tube and inside of the pipet. For each analytical method and type of sample the validity of the sampling procedure must he verified experimentally.
lO,OO0
ABSORBER MG/SQ CM
I
I 240
Figure 2.
480
720
Composite Aluminum Absorption Curve
Mixture of Ag”’ (1.0 m.e.v.) and Ru’06 (3.1 m.e.v. effective @-energy), showing resolution into individual components
it is possible to estimate the contribution of the stronger P-emitter by ext,rapolation and subtract it from the total, the difference gives the contribution of the weaker constituent (Figure 2). Similarly, if two or even more constituents are prpsent with clearly differin half-lives, it may be possible to resolve the mix8 ture by allowing the short-lived components to decay away, leaving only the longest lived isotope, the half-life of which is easily determined. The contribution due to this isotope may be calculated for various times and subtracted from the total, leaving the contributions of the others. The next longest lived may be identified in the same way and subtracted in turn to give the next component. An example is shown in Figure 3. Special cases exist where not only qualitative identification but quantitative analysis of simple mixtures is possible if the composition is known to be limited to two or a t the most three familiar constituents. iln example of the application of &absorption measurements to this problem is the determination of cerium activity in yttrium tracer preparations made from uranium fission, which may be done very easily by placing enough absorber between the sample and the counter to cut out,yttrium radiations. Any count observed would then be due to cerium impurity (Figure 1). From the relative comting rates of a pure cerium sample without whsorher and with absorber sufficient to stop the yttrium prays, the contribution of the cerium t o the total counting rate of the yttrium sample of zero absorber can be calculated. If a sample is fairly complex, or if the radiation characteristics are insufficiently different to permit this sort, of simple physical analysis, chemical separation of the different radioactive elements or compounds is necessary. QUANTITATIVE ACTIVITY ANALYSIS
The general procedure for the quantitative radiochemical analysis of a complex mixture involves: (1) sampling, (2) addition
I
\ \I
I00
16
Figure 3. Mixture of 8d
I131
32
48
64
80
DAYS
Composite Decay Curve
and 32d Te‘*, showing resolution into individual components
Carrier. A known amount of the ordinary inactive isotope or isotopes of the element to be determined is added to serve as a “carrier” for the radioactive atoms and to give them the properties characteristic of the element in ordinary amounts, rather than the often unusual properties associated with estr~niely low concentrations. In general, one does not strive for a qu.intitative recovery of the carrier but is content with recoveling a substantial portion of it in a pure state and correcting his wsult for the amount lost. This has the obvious advantage of prrmitting rapid work without taking pains to make recovery quanti-
324 tative. Any separation may be used which yields a pure product, even though the recovery may be far from complete. This is an important advantage if speed is essential, especially with the more difficult elements such as columbium, zirconium, and the rare earths. It is possible, for example, to determine columbium activity quantitatively in the presence of zirconium and the rare earths in about one hour. Obviously, if the original sample already contains significant amounts of the element or substance to be determined, the ordinary carrier method cannot be used. The constituent in question must be isolated quantitatively in the activity analysis or the total amount determined before the separation process is begun. The carrier method is based on the assumption that tracer and carrier always have exactly the same properties. This is generally true, but certain exceptions must be noted. If carrier and tracer are in different chemical states-e.g., iodide and iodate in alkaline solutions-and these do not interact with each other (exchange), subsequent chemical treatment may separate the active and inactive atoms and give an entirely erroneous result. It is therefore necessary to ensure that active and inactive atoms all reach the same chemical state before any separation step is applied. An example of exchange difficulty can be found in the determination of tellurium activity. If the radioactive tellurium is present as tellurate and the carrier is added as tellurite, the rate of exchange between the two is ordinarily slow. Precipitation of the tellurite is metallic tellurium by reduction with sulfur dioxide in hydrochloric acid removes it quantitatively, but most of the tellurate tracer is unaffected and remains in solution Hence, the first step in activity analysis is frequently a rather drastic one chosen to convert tracer and carrier to the same chemical state. For certain elements such as technecium, plutonium, and promethium, no inactive isotopes exist for use as carriers. Recourse must then be made either to the use of a nonisotopic but chemically very similar carrier-for example, another rare earth in the determination of promethium-or to rigid quantitative isolation of the radioelement directly. The latter is ordinarily difficult, but possible if suitable precautions are taken. Chemical Separations. Subsequent chemical separations are designed with two objectives in mind: radiochemical purity of the tracer and chemical purity of the carrier. One must be certain that all interfering radioactivities are removed during these operations and it must be remembered that if the impurities are present in very low chemical concentrations, their chemical behavior may be different from that observed when they are present in macroamounts. This difference may be either to the advantage or disadvantage of the analyst. Experiment determines the number of individual steps necessary to give adequate decontamination. Evidently the determination of small amounts of active impurities in a relatively pure tracer preparation requires better decontamination than the determination of the major constituent. The achievement of satisfactory chemical purity in the recovery of the carrier is usually not difficult. Isolation of Carrier and Determination of Chemical Yield. Khen satisfactory chemical and radiochemical punty has! been achieved, it is necessary to determine the radioactivity of the final product and the completeness of recovery. A saving in time is possible if the radioactivity and chemical measurements can both be done with the sample in the same physical state. Radioactivity is usually determined in solid samples and for this reason the chemical yield is most often determined gravimetrically by weighing the find sample just before or after counting it. With the development of satisfactory liquid counters it is likely that more frequent use will be made of colorimetric and volumetric methods for the determination of chemical yields. For most samples which are counted as solids, determination of the chemical yield with an accuracy of *2% is sufficient, as the radioactive measurement is not so good as that. If the yield is to be determined gravimetrically, the mounted
ANALYTICAL CHEMISTRY -ample must be pure and of known composition. If volumetric or colorimetric determination of the chemical yield is used, only those elements which would constitute interferences need be removed. Counting. Surprisingly enough, the measurement of the radioactivity of the isolated sample is frequently the most inaccurate and unsatisfactory step in the entire analysis. This is particularly true if the radioisotope is a @-emitterand is counted by placing the solid sample near a Geiger tube-the most common method of counting. Information is widely available on the magnitude of the error produced through statistical fluctuations in radioactive decay (8), and tables and graphs have been prepared showing how long it is necessary to count to get 10, 1, or 0.1% precision. The ordinary errors inherent in the use of Geiger tubes are also well known-namely, the effect of coincidence losses a t high counting rates, and the unpredictable fluctuations in counter efficiency which are also observed (8). Unfortunately, many workers have not realized that these effects are virtually negligible in comparison with the differences that may arise in 8-counting rates through small differences in the physical form of the mounted sample. Several important factors enter into the apparent counting rate of a solid sample of the sort a chemist is usually interested in, mounted in the conventional r a y under an end-window Geiger tube. These include geometry factor, absorption due to air, window, cover, and the sample itself, and the efficiency with which the material on which the sample is placed reflects (scatters) donmvard-directed 8-particles back upward into the counter. Beyond these is one other very important factor: the efficiency with which the sample itself scatters its own radiations. This phenomenon, known as selfscattering, is the major obstacle to accuracy with this type of counting
1.00
1
I 20
I I I 30 40 50 SAMPLE WEIGHT MG. F i g u r e 4. T y p i c a l S e l f - s c a t t e r i n g Curve f o r 8E m i t t e r of A b o u t 1.5 3I.E.V. Energy Mountingu
I IO
.
watch glasses in atandard counting arrangement
EFFECTOF PHYSICAL FORMOF SAMPLE.The effect of the physical form of the mounted sample on the counting rate has been described in an important paper by Engelkemeir, Seiler, Steinberg, and Winsberg, which has for several years been awaiting publication as part of the Plutonium Project Record (3). The problem was studied independently by Hume, Sadowski, and hlahlman (7) a t the same time as the other workers, and the report of these results is likewise awaiting appearance of the National Nuclear Energy Series. The essential features of these studies are summarized below. If identical small portions of an essentially carrier-free tracer solution are evaporated in the same small area in the centers of a series of uniform metal plates or in small metal cups and these samples are counted, good results are obtained. But if to each of these samples is added during evaporation a significant amount (5 to 50 mg.) of solid material, the counting rates of the different samples no longer agree, If a plot is made of the observed counting rate of the fixed amount of tracer evaporated with varying amounts of carrier against the amount of carrier used, a curve which frequently has the form shown in Figure 4 is obtained.
'
V O L U M E 2 1 , NO. 3, M A R C H 1 9 4 9 The counting rate increases with increasing carrier, passes through a maximum, and then decreases, owing to self-absorption. The increase over the zero carrier count may be as high as 30%. Individual points on the curve reproduce poorly; 10% differences are not uncommon, although the order of magnitude of the effect is never in doubt. A curve obtained not by evaporation but by precipitation of increasing amounts of carrier with the tracer and filtering and mounting the precipitates will have somewhat better reproducibility. The scattering effect is dependent upon the physical form and particularly upon the density of the solid substance as well as the energy spectrum of the radioisotope. The counting of solid samples is therefore highly uncertain unless all in a series have exactly the same physical form and the total weight is always about the same. I t is not possible to compare a sample evaporated on a plate with another in the form of a precipitate transferred to a similar plate. The author has observed repeatedly, for example, that a pure yttrium tracer evaporated to dryness on a watch glass with 20 mg. of yttrium carrier as the nitrate gave results as much as 20y0 below those obtained when the activity was precipitated as the oxalate with the same amount of yttrium carrier, filtered, and transferred to a watch glass for counting. A similar esperience was encountered with a sample of ruthenium tracer. Samples of the tracer were evaporated with added ruthenium chloride as carrier in porcelain counting dishes of the sort frequently used in biological work. After counting, the samplesstill in the same dishes-were ignited in hydrogen to reduce the carrier ruthenium to the metallic state. On re‘measurement, the counting rates were found to be approximately 10% higher. .4n examination of the literature reveals that successful tracer applications have a h a y s involved a rigid standardiiation of counting conditions. For the research worker using a single isotope and able, in general, ‘to set his counting conditions as he wishes, this does not involve any great difficulty. For the analyst n h o must try to make sense out of results obtained on a sample “as received,” the lack of a uiiiversallp applicable method of preparing samples for comparative counting is a major obstacle. The direct counting of @-activityin solutions has received comparatively little attention in the past, but the obvious advantages involved make the method appear attractive in spite of difficulties due to self-absorption and contamination of the counting apparatus. I t is not unlikely that liquid counting may ultimately surpass solid counting in popularity for radioisotopes that emit moderately strong @-rays. The measurement of or-activity involves difficultiesalso, but these are primarily due to absorption and are well understood. Comparatively little work is done with a-emitting isotopes in comparison to that which is done with 8- and y-emitters and therefore no extensive discussion is given here. The measurement of y-activity is in some respects much simpler than the measurement of @-activity,since y-rays do not scatter and the physical state of the sample is of relatively little consequence. Good precision can be obtained in the measurement of y-radiation by several different methods-Geiger tubes, electroscopes, high pressure ionization chambers, and scintillation photomultipliers, to mention only four. The difficulty lies in the fact that the observed result is always dependent upon the nature of the apparatus used and it is extremely difficult, without direct experimental comparison, to correlate the results obtained by one method with those obtained by any of the others. Further than that, comparatively minor variations in the apparatus produce considerable differences with any one of the methods of measurement. In most laboratories y-activity is measured with an ordinary Geiger tube by inserting between the sample and the tube an absorber sufficiently thick to stop all @-particles. The y-rays themselves do not produce counts but instead eject Compton and photoelectrons from the absorber or the tube itself and these par. ticles are what is detected by the Geiger tube. The counting
325 cbfficiency depends on the tube, the nature of the absorber used, and the energy of the y-ray. On the Manhattan Project, there were a t least three absorbers in common use for y-counting: a slab of aluminum of thickness equivalent to 1.7 grams per sq. em., a sheet of lead with a thickness equivalent to 1.9 grams per sq. em., and a %mdv,-ich” consisting of a 1.9 gram per sq. cm. sheet of lead with 1.7 grams per sq em. of aluminum underneath and a sl~eetof about 130 mg. per sq. cm. of aluminum on top. The sandwich, which is the invention of iT. H Sullivan, is based on the following principles: The thick aluminum takes out the @-particles but with the production of soft x-rays (Bremsstrahlen),the lead serves to filter out the x-rays, and the top aluminum sheet eliminates photoelectrons emitted from the lead. This leaves, theoretically, only Compton electrons from the top aluminum sheet to be counted and the counting rate should be proportional to the intensity of the y-rays. The counting rate with such an arrangement should have a closer correlation with the actual y-dirintegration rate than the counting rate obtained with the simpler absorbers. The results obtained by counting the same aaniples with thr different absorbers vary greatly. The magnitude of the variation is dependent upon the energy of both the -pray and the p r a y s being absorbed. Typical results are given in Table I, where the counting rates of samples of three tracers are compared under each of the three standard counting conditions. An analyst nho found 1.0% y-activity due to ceiiuin 1)y the sandwich method TI ould not agree with another analyst n ho u v d aluminum alone, for the latter n-ould report 5.47,! There is J, rcal need for wideiprrad qtandardization of y-counting conwntion- 2nd intercompaiiwn of various y-counting mctliodj -
-
‘Table I.
Comparison of Kelati\e y-Counting Rates with Different @-Ra>Absorbers Radioisotope Sandwlch Pb 1 9 B /Si C m \I 1 i G /Sq C m
-
Cbgj ce1r1-4
La’ll
(1.00t
2.0i 4.07 1.63
(1.00) (1.OO)
1.49
.i,42
1.49
For each isotope counting rates are mworted in tvr:!ls of ratio of observed rate t o rate obtained with sanrlwichi ~~~
~_
~
- ~.
.~ ..___
Calculations. If a sample is counted in the usual manner, the observed counting rate is corrected for background and normalized to standard counter efficiency. h correction may also be applied for self-absorption of the sample. If the radioisotope is short-lived or undergoes rapid daughter groxth, the observed counting rate may need to be corrected to some standard comparison time by the usual equations of radioactive decay. The final count is then corrected for losses during the chemical operations.
A concrete illustration might be the determination of P 3 2 8activity in a mixture of carrier-free phosphorus, cobalt, and cesium tracers which had a total radioactivity of 4000 counts per minute per ml. a t 8:OO A.M. Thursday. To 5.00 ml. of solution was added phosphorus carrier equivalent to 54.0 mg. of magnesium ammonium phosphate hexahydrate. After appropriate chemical steps, 42.7 mg. of magnesium ammonium phosphate were recovered, chemically and radiochemically pure. At noon on Friday this sample counted under standard conditions 5000 counts per minute. Corrected for the chemical yield of 79%, the count was 6330 counts per minute of phosphorus in the 5-ml. sample. Referred back to 8 : O O A . V . the previous day by allowing for decay of the 14.3~1P32the count would have been 6700 counts per minute, or 1340 counts per minute per ml.: in other words, 33.570 of the total. - i t other times, because the several isotopes in the mixture decay with differing rates, the percentage composition would not be the same. APPLICATIONS
During the war, the research and development program of the Plutonium Project required an unprecedented number and vari-
ANALYTICAL CHEMISTRY
326 ety of radiochemical activity analyses for the fission products of uranium. The bulk of the research papers by a large number of workers, principally under C . D. Coryell, K. Sugarman, and D. ? Hume, i.is scheduled for publication in Volume 9B of t h e Plutonium Project Record. A brief review of the methodology and a few examples are given here, as the work provides a particularly apt illustration of the general principles involved. t
Table 11.
-
Flowsheet of Radiochemical Activity Analysis for Zirconium
I
Sample. Add HXOa, Zr carrier, and HF. Add La carrier Ppt. LaFa plus Soln. U, Zr, fission products. Add Ba carrier rare earths Ba Ppt. BaZrFe. Dissolve in HNOa and U , fission HIBOZ. Reprecipitate. Redissolve ux sozoducts and add H ~ S O I 8r activities (Diarard) (Discard) Soln. ZrO+’. Ppt with cupferron, ignite to ZrOz, and weigh. (Discard) Measure radioactivitv
1
~
i
I
Procedures were developed for the rapid determination of the activity due to each of the fission elements, other than the noble gases and certain rare earths, in the very complex mixtures of activities resulting from uranium fission. The general procedure for each element was as outlined above. Procedures were developed in which liquid samples up to 5 or 10 ml. were used and most operations were conducted in 50-mi.
centrifuge tubes. Centrifugation rather than filtration was the preferred method of separation and in many respects the technique was similar to that of modern qualitative semimicroanalysis. Samples were chosen such that in general no health hazard due to radiation was encountered. Samples were invariably mounted on 2.5-cm. (1 inch) watch glasses for counting. Liquids were evaporated on the watch glasses togethemith enough solid material (usually cerous nitrate) to get a total weight of about 30 mg. to provide optimum self-scattering. A t the conclusion of the analysis, the purified constituent was precipitated preferably in a form which could be filtered out on a previously weighed 2.2-cm. (0.875-inch) filter paper in a Hirsch funnel, dried with alcohol and ether or in an oven a t 110” C., and weighed. The weighed precipitate together with its filter paper was then transferred to a watch glass for counting. Precipitates that required ignition were ignited in crucibles, weighed by difference onto watch glasses, and spread smoothly. The watch glasses were mounted on stiff 2.5 X 3.25 inch cards designed to fit the standard counting apparatus. A typical example of activity analysis as applied to fission product mixtures is the determination of zirconium activity described below (6). The outline of the final chemical procedure is shown in Table 11. The sample of mixed h i o n product radioactivities is measured out into a 50 ml. Lusteroid centrifuge tube, and zirconium carrier and an excess of nitric and hydrofluoric acids are added. This serves to dissolve the zirconium as the very stable fluoride complex. It had been found that direct precipitation of zirconium as zirconium phosphate a t this point resulted in the loss of much of the zirconiumactivity. Addition of oxalic acid to convert both tracer and carrier zirconium to the same form (oxalate complex) did not entirely correct the difficulty, but treatment with hydrofluoric acid served to ensure complete exchange between carrier and tracer. This was checked by comparative analyses and by ey eriments with pure tracers &he first separation step involves the removal of rare earths together with much of the alkaline earths and uranium-X, if present. Lanthanum nitrate is added as a carrier for these elements. Upon the formation of a precipitate of lanthanum fluoride, rare earth and uranium-X radioactivities are carried down as insoluble fluorides. Barium and strontium, although soluble when present in macroamounts, are strongly coprecipitated by lanthanum fluoride when they are present in low concentrations, and they are removed also. The rare earth precipitate is centrifuged out, the solution is decanted into another tube, and the original tube together with ita precipitate is thrown away. A second or third scavenging by addition of more lan-
thanum carrier may be desirable if the ratio of zirconium activity to rare earth and alkaline earth activities is low. The next step involves the precipitation of the zirconium as barium fluozirconate for the separation from the other fission elements, especially columbium. Methods available in the literature for the separation of zirconium and columbium were found insufficiently complete or unsatisfactorily time-consuming. The separation by precipitation as barium fluozirconate offers as usually good separation of columbium from zirconium. .4lthough the precipitate is fairly soluble, the yield is adequate, and the radiochemical purity is very high. Tracer experiments showed three precipitations to decrease the columbium activity contamination by a factor of lo6 and the removal of the other fission products was even better. The barium fluozirconate precipitate is nicely crystalline, comes down readily and rapidly, and is very easily dissolved afterwards in a mixture of boric and nitric acids. The boric acid serves to remove fluoride from solution as the stable fluoborate ion and is a very convenient means of aiding the solution of insoluble fluorides in general. By this time the zirconium is essentially pure radiochemically. Before it can be precipitated as the cupferride for filtering and ignition to the oxide, the relative1 high concentration of inactive barium present must be removeJ by precipitation with sulfate. Unless this is done, barium contaminates the final oxide sample and gives a result for the chemical yield which is several per cent too high. After the zirconium oxide has been weighed, it is transferred to a watch glass and the p- or 7-activity is counted in the usual way. This procedure was tested by analysis of pure zirconium-free tracer preparations of all the common fission elements to demonstrate that those radioactivities did not come through the procedure. It was also tested by analyzing unseparated fission product mixtures and demonstrating that no significant radioactive impurities were present in the final zirconium precipitate. The procedure described can easily be done in duplicate in less than 2 hours, and similar speed was found attainable with most of the other fission elements. The major sources of error were in sampling and counting. Results good to *5% were usually satisfactory for the purposes in hand and only in unusual circumstances was an effort made to obtain accuracy higher than 2 to 3oJ,. I t is anticipated that as more radioactive tracers from pile and accelerator sources are marketed, many more standard methods of activity analysis will be developed and put into use. The existence of a reliable activity analysis procedure for a given element makes it possible to do tracer experiments with that element without first isolating the pure radioactivity. An unseparated mixture of carrier-free activities may be employed in the experiment and the samples analyzed for the particular element or elements of interest. This may be of considerable importance where tracer preparation is long and difficult, as the analysis procedure is usually comparatively short and simple. ACKNOWLEDGMENTS
The experimental work of this paper was performed a t the Clinton Laboratories, Oak Ridge, Tenn. The author is indebted to the Officeof Naval Research and the Laboratory for Suclear Science and Engineering, Massachusetts Institute of Technology, for partial support. LITERATURE CITED
(1) Borkowski, C. J., ANAL.CREW.,21, 348 (1949). (2) Clark, H. M . , and Jones, J. W., Nucleonics, 2 (5), 25 (1948). (3) Engelkemeir, D. W., Seiler, J. A., Steinberg, E. P., and Winsberg, (4)
(5) (6)
(7)
(8)
L., “National Nuclear Energy Series, Plutonium Project Record,” Vol. 9B (publicationpending). Feather, N., Proc. CambridgePhil. Soc., 34,599 (1938). Glendenin, L. E., Nucleonics, 2 (11, 12-32 (1948). Hume, D. N., “Plutonium Project Record,” 1’01. 9B (publication pending). Hume, D. N., Ballou, X. E., and Glendenin, L. E., “Manual of Radiochemical Determination of Fission Product Activities,” Clinton Labs. Rept. CN-2815 (1945) ; “Plutonium Project Record,” Vol. 13C (publication pending), Kohman, T. P., ANAL.CHEM.21,352 (1949).
R~CEIYE January D 11, 1948.
